I post the solution i got from Ext.Net support team so if someone need it in future:
@(Html.X().ComboBox()
.Listeners(events => {
events.Expand.Handler = @"this.listKeyNav.map.removeBinding({
key: Ext.EventObject.ENTER
});";
events.Expand.Delay = 1;
events.Expand.Single = true;
})
)